As an aluminum support for printing plate, particularly for offset printing plate there is used an aluminum plate (including aluminum alloy plate).
In general, an aluminum plate to be used as a support for offset printing plate needs to have a proper adhesion to a photographic light-sensitive material and a proper water retention.
The surface of the aluminum plate should be uniformly and finely grained to meet the aforesaid requirements. This graining process largely affects a printing performance and a durability of the printing plate upon the printing process following manufacture of the plate. Thus, it is important for the manufacture of the plate whether such graining is satisfactory or not.
In general, an alternating current electrolytic graining method is used as the method of graining an aluminum support for a printing plate. There are a variety of suitable alternating currents, for example, a normal alternating waveform such as a sinewaveform, a special alternating waveform such as a squarewaveform, and the like. When the aluminum support is grained by alternating current supplied between the aluminum plate and an opposite electrode such as a graphite electrode, this graining is usually conducted only one time, as the result of which, the depth of pits formed by the graining is small over the whole surface thereof. Also, the durability of the grained printing plate during printing will deteriorate. Therefore, in order to obtain a uniformly and closely grained aluminum plate satisfying the requirement of a printing plate with deep pits as compared with their diameters, a variety of methods have been proposed as follows.
One method is a graining method to use a current of particular waveform for an electrolytic power source (JP-A-53-67507). (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) Another method is to control a ratio between an electricity quantity of a positive period and that of a negative period at the time of alternating electrolytic graining (JP-A-54-65607). Still another method is to control the waveform supplied from an electrolytic power source (JP-A-55-25381). Finally, another method is directed to a combination of current density (JP-A-56-29699).
Further, known is a graining method using a combination of an AC electrolytic etching method with a mechanical graining method (JP-A-55-142695).
As the method of producing an aluminum support, on the other hand, known is a method in which an aluminum ingot is melted and held, and then cast into a slab (having a thickness in a range from 400 to 600 mm, a width in a range from 1,000 to 2,000 mm, and a length in a range from 2,000 to 6,000 mm). Then, the cast slab thus obtained is subjected to a scalping step in which the slab surface is scalped by 3 to 10 mm with a scalping machine so as to remove an impurity structure portion on the surface. Next, the slab is subjected to a soaking treatment step in which the slab is kept in a soaking furnace at a temperature in a range from 480.degree. to 540.degree. C. for a time in a range from 6 to 12 hours, thereby to remove any stress inside the slab and make the structure of the slab uniform. Then, the thus treated slab is hot rolled at a temperature in a range from 480.degree. to 540.degree. C. to a thickness in a range from 5 to 40 mm. Thereafter, the hot rolled slab is cold rolled at room temperature into a plate of a predetermined thickness. Then, in order to make the structure uniform and improve the flatness of the plate, the thus cold rolled plate is annealed thereby to make the rolled structure, etc. uniform, and the plate is then subjected to correction by cold rolling to a predetermined thickness. Such an aluminum plate obtained in the manner described above has been used as a support for a planographic printing plate.
However, electrolytic graining is apt to be influenced by an aluminum support to be treated. If an aluminum support is prepared through melting and holding, casting, scalping and soaking, even though passing through repetition of heating and cooling followed by scalping of a surface layer, scattering of the metal alloy components is generated in the surface layer, causing a drop in the yield of a planographic printing plate.
In this connection, the present inventors have previously proposed a method of producing a support for planographic printing plate, which comprises continuously performing casting and hot-rolling from molten aluminum to form a hot-rolled coil of a thin plate, transforming the hot-rolled coil into an aluminum support through cold-rolling, heat-treatment and correction, and finally, graining the aluminum support (U.S. Pat. No. 5,078,805 which corresponds to JP-A-3-79798).
However, even the preparation methods which have been previously proposed by the present inventors give the non-uniformity of the yield of electrolytic graining and the graining property due to the components of aluminum support.
Further, in order to prepare an aluminum alloy having the foregoing composition, a method is normally employed which comprises melting an ingot having an aluminum content of not less than 99.7%, and then adding an aluminum mother alloy containing predetermined amounts of Fe, Si and Cu to the molten aluminum. This aluminum mother alloy is expensive as compared with an aluminum ingot, raising the cost of aluminum alloy.